1. Field of the Invention
The present invention relates generally to a semiconductor unit having two or more types of electric source systems.
2. Description of the Prior Art
Hereinafter, a conventional semiconductor unit is described with reference to drawings. There are many types of semiconductor units which have two or more types of electric source systems separated from each other. As a representative unit, a semiconductor unit having a digital section and an analog section is known. In general, voltages applied by the different electric source systems in one semiconductor unit differ from each other. However, in at least one case the voltages applied by the different electric source systems in one semiconductor unit are the same.
An example of a conventional semiconductor unit shown in FIG. 1 has an analog section as a first circuit and a digital section as a second circuit. The analog section includes an analog circuit 1, an output circuit 3 and an input protective circuit 5. The digital section includes a digital circuit 2 and an input circuit 4. The first and second circuits are connected with each other through a signal line S1 between the output circuit 3 and the input circuit 4 of the respective first and second circuits.
An electric source voltage applied to the analog section is supplied through electric source lines Vdd1 and Vss1, and an electric source voltage applied to the digital section is supplied through electric source lines Vdd2 and Vss2. The electric source line Vdd1 is separated from the electric source line Vdd2, and the electric source line Vss1 is separated from the electric source line Vss2.
In the above configuration, a signal is output from the analog circuit 1 through the output circuit 3 to the digital section. Thereafter, the signal is input to the input circuit 4, an inverter, through the signal line S1 and is transmitted to the digital circuit 2.
Also, another conventional semiconductor unit shown in FIG. 2 has an analog section composed of an analog circuit 11, an output circuit 13 and an input protective circuit 15 and a digital section composed of a digital circuit 12 and an input circuit 14. The first and second circuits are connected with each other through a signal line S1. An electric source voltage applied to the analog section is supplied through electric source lines Vdd and Vss, and an electric source voltage applied to the digital section is supplied through the same electric source lines Vdd and Vss. In the above configuration, the path for the electric source line Vss takes a long way around because of a design requirement and the electric source line Vss becomes lengthy.
In these prior art circuits, certain surges applied to input terminals NT or to a low potential side terminal Vss1 or Vss cause semiconductor units to fail in spite of the protection provided by the input protection circuit 5 or 15.